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2016 ; 14
(4
): e1002427
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Excessive Osteocytic Fgf23 Secretion Contributes to Pyrophosphate Accumulation
and Mineralization Defect in Hyp Mice
#MMPMID27035636
Murali SK
; Andrukhova O
; Clinkenbeard EL
; White KE
; Erben RG
PLoS Biol
2016[Apr]; 14
(4
): e1002427
PMID27035636
show ga
X-linked hypophosphatemia (XLH) is the most frequent form of inherited rickets in
humans caused by mutations in the phosphate-regulating gene with homologies to
endopeptidases on the X-chromosome (PHEX). Hyp mice, a murine homologue of XLH,
are characterized by hypophosphatemia, inappropriately low serum vitamin D
levels, increased serum fibroblast growth factor-23 (Fgf23), and osteomalacia.
Although Fgf23 is known to be responsible for hypophosphatemia and reduced
vitamin D hormone levels in Hyp mice, its putative role as an auto-/paracrine
osteomalacia-causing factor has not been explored. We recently reported that
Fgf23 is a suppressor of tissue nonspecific alkaline phosphatase (Tnap)
transcription via FGF receptor-3 (FGFR3) signaling, leading to inhibition of
mineralization through accumulation of the TNAP substrate pyrophosphate. Here, we
report that the pyrophosphate concentration is increased in Hyp bones, and that
Tnap expression is decreased in Hyp-derived osteocyte-like cells but not in
Hyp-derived osteoblasts ex vivo and in vitro. In situ mRNA expression profiling
in bone cryosections revealed a ~70-fold up-regulation of Fgfr3 mRNA in
osteocytes versus osteoblasts of Hyp mice. In addition, we show that blocking of
increased Fgf23-FGFR3 signaling with anti-Fgf23 antibodies or an FGFR3 inhibitor
partially restored the suppression of Tnap expression, phosphate production, and
mineralization, and decreased pyrophosphate concentration in Hyp-derived
osteocyte-like cells in vitro. In vivo, bone-specific deletion of Fgf23 in Hyp
mice rescued the suppressed TNAP activity in osteocytes of Hyp mice. Moreover,
treatment of wild-type osteoblasts or mice with recombinant FGF23 suppressed Tnap
mRNA expression and increased pyrophosphate concentrations in the culture medium
and in bone, respectively. In conclusion, we found that the cell autonomous
increase in Fgf23 secretion in Hyp osteocytes drives the accumulation of
pyrophosphate through auto-/paracrine suppression of TNAP. Hence, we have
identified a novel mechanism contributing to the mineralization defect in Hyp
mice.